The first study to examine the probability of HIV infection per act
of heterosexual sex among a population with multiple sexual partners has
found that uncircumcised men have more than twice the risk of acquiring
HIV than do circumcised men.

In the study, published in the Feb. 15 issue of The Journal of
Infectious Diseases, now available online, Jared Baeten and colleagues
from the United States and Kenya collected detailed sexual data from a
group of male Kenyan truckers and, using statistical models, developed
infectivity measures that estimate the per-sexual-act probability of HIV
transmission. The study is the first to calculate the probability of
infection for men who have multiple, concurrent heterosexual partners,
which was found to be significantly higher than infectivity rates
calculated in the past from studies of monogamous couples. Their results
may help explain the rapid spread of HIV in settings where circumcision
is not common and multiple sexual partnerships are.

Between 1993 and 1997, 745 male employees of trucking companies based
in Mombasa, Kenya were followed for the study. Initially they were
evaluated for circumcision status and HIV-negativity. Over the length of
the study the men were asked to give information concerning the number
of sexual encounters with three different partner types--wives, casual
partners, and prostitutes--and were screened for HIV and other sexually
transmitted infections. At the end of the study the probability of
infection was calculated using a statistical model that incorporated
published data to estimate the rates of HIV infection among the three
types of sexual partners.

For the men in the study, the overall probably of becoming
HIV-infected following a single act of intercourse was calculated to be
.0063, or one in 160. Uncircumcised men had a more than two-fold
increased risk of infection per sexual act compared with circumcised
men--one in 80 versus one in 200. Past studies have also found greater
HIV infection risk for uncircumcised men. Unlike those studies, however,
the present study was also able to take into account cultural
characteristics that might be responsible for differences in sexual
behavior. These differences could, in turn, account for differences in
the risk of infection. But the researchers found that cultural
differences in sexual behavior did not matter: when groups of men were
excluded from the analysis based on ethnic or religious characteristics,
the difference in probability of infection related to circumcision
status did not change.

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In addition to the results on circumcision, another significant
result of the study was the high overall rate of per-contact infection.
In past studies that attempted to calculate the probability of
female-to-male infection through heterosexual sex, the cohorts consisted
only of monogamous couples in which the female partners were
HIV-positive. However, in some areas of Africa where HIV infection rates
are highest, multiple, concurrent partnerships are more common than
monogamous couplings. Past studies of monogamous couples found that the
probability of HIV transmission per-act of sexual intercourse was on the
order of one in 1000 or less, much lower than the probability of one in
160 found in this study. Taken with the authors' evidence that
female-to-male infection rates may be more than two-fold greater for
uncircumcised men, this new information may help us understand why the
virus is spreading rapidly in parts of Africa, and help define better
HIV strategies for prevention in these settings.